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Fact Sheet
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Skin and Respiratory Allergies
Allergies are often performance-limiting, painful, and unsightly—not to mention expensive to diagnose and treat.
Overview
Signs of Allergies
Allergic reactions range from mild reactions affecting a small region of the skin to
life-threatening (e.g., anaphylactic) reactions. Common signs of skin allergies include itching, rubbing, and scratching, urticaria (i.e., transient focal swellings in the
skin or mucus membranes called hives),
thickened skin, and hair loss.1,2
Signs of respiratory allergy include
coughing, labored breathing, exercise intolerance, and poor performance. Allergic
airway disease is also referred to as heaves,
location and appearance of the lesions or
rash.4
Biopsy of lesions and routine microscopic examination of the sample can help rule
out other causes of skin problems. Some
conditions mimicking skin allergies include
reactions to heat, cold, or light (photosensitization, sunburn), infections, neoplasia,
lungworms, or even psychogenic causes.2,4
Allergy Testing
DUSTY PERIN
An allergy is a hypersensitivity to an allergen, which is any substance that can
stimulate the immune system such as parasites, bacteria, viruses, etc. When a horse is
exposed to an allergen, the horse’s immune
system produces antibodies called immunoglobulin E (IgE).
Normally, only a limited amount of IgE is
produced that “primes” the immune system
in case the horse comes into contact with
that same (potentially dangerous) allergen
again. In a horse with allergies, however,
the horse’s immune system makes an abnormally large quantity of IgE to “harmless”
antigens, such as mold spores, pollens, and
insect bites. After being repeatedly exposed
to these harmless allergens, the allergens
bind to the IgE molecules that coat the surface of special cells of the immune system
called mast cells and basophils. Once stimulated, these cells release a large amount of
inflammatory mediators (e.g., cytokines,
histamine). These inflammatory mediators
are extremely potent and cause classic signs
of allergies due to smooth muscle constriction, dilation of blood vessels, and stimulation of the nervous system.1
Equine allergies, which primarily affect
the skin and respiratory tract, are increasingly common. Horses can develop allergies
at any age and remain allergic to those substances for the rest of their lives.
Allergies primarily affect the skin and respiratory
tract.
recurrent airway obstruction (RAO), and
inflammatory airway disease (IAD).3
Potential Allergens in Horses
Some of the most common causes of allergies in horses include:
■ Insect bite hypersensitivity from biting
midges (Culicoides), as well as biting
gnats, horse flies, house flies, and mosquitoes;
■ Mites (chorioptic and psorioptic mange
and trombiculidiasis or “chiggers”);
■ Grass, weeds, and tree pollens;
■ Horse feed ingredients or food additives;
■ Mold spores;
■ Vaccines and drugs; and
■ Animal danders, dust, feathers, aerosols,
and volatile chemicals.
A contact dermatitis can be caused by direct contact with an irritant such as a bit,
saddle, or other tack item.2,4,5
Diagnosing Allergies
Skin allergies can only be diagnosed after
all other known causes of itchiness (pruritus) have been ruled out by a thorough
physical examination and history. Important facts your vet will consider are where
the horse resides (stable or pasture), age of
onset, seasonality of the condition, and the
Two types of allergy tests are currently
available: intradermal allergy testing (IDT)
and serum allergy testing (SAT). These tests
help identify allergens to avoid exposure and
help choose allergens for allergen exposure
immunotherapy (ASIT).
The IDT involves injecting small amounts
of aqueous allergen extracts into the skin
(usually on the neck). The injection sites are
assessed 15–30 minutes later (for immediate reactions) and between 4 and 24 hours
(for delayed reactions). A positive response
is the formation of a wheal (a raised flat-top
swelling) at the injection site. The use of
corticosteroids, antihistamines, or phenothiazine tranquilizers is known to interfere
with the IDT, so these products must not be
administered to the horse for about two to
four weeks before testing, which could prove
difficult in an itchy horse.
Serum allergy testing (SAT) involves submitting a single blood sample to a laboratory
that will then look for IgE antibodies against
common allergens. The SAT is not impacted
by the use of antihistamines.
Both tests can generate false positives and
negatives, and multiple factors can impact
the test results (even excitement). Thus, the
results of the IDT and SAT, even negative
results, must be considered with the horse’s
history and physical exam findings.2,4,5
Treatment
A multimodal treatment approach is encouraged for treating equine allergies.1–5 First,
use blankets or pyrethrin-impregnated fly
This Fact Sheet may be reprinted and distributed in this exact form for educational purposes only in print or electronically. It may not be used for
commercial purposes in print or electronically or republished on a Web site, forum, or blog. For more horse health information on this and other topics visit
TheHorse.com. Published by The Horse: Your Guide To Equine Health Care, © Copyright 2010 Blood-Horse Publications. Contact [email protected].
Fact Sheet
sheets on horses that are hypersensitive to culicoides (sweet itch), stable them during peak
insect activity (early morning and evening),
and use bug repellents, screens, and fans.
Second, consider using soothing shampoos
and conditioners, and if necessary, topical
corticosteroids. Third, corticosteroids are often used in horses with dermatologic (skin)
allergic manifestations. Prednisolone given
daily (orally) is expected to induce clinical
remission. The dose can be tapered slowly
over time to the lowest dose that will control
the urticaria. Prednisolone is preferable to
other corticosteroids such as dexamethasone
or triamcinolone as prednisolone is less immunosuppressive and less likely to result in
laminitis. Glucocorticoids exert profound effects upon all organ systems; long-term use
can lead to an unacceptably high risk of serious side effects, compared with any possible
benefits.
Other agents that can be useful in managing urticaria are antihistamines, tricyclic antidepressants, phosphodiesterase inhibitors,
diethylcarbamazine, levamisole, and cyclosporine, as well as nutritional supplements
such as methylsulfonylmethane, antioxidants, and digestive support.2,6,7
Hyposensitization, also referred to as allergen-specific immunotherapy (ASIT), has
been used to manage urticaria, insect hypersensitivities, and allergen-induced RAO. Allergens are combined in solution and injected
under the skin to induce immunologic tolerance. If effective, treatment is usually lifelong.
8 Response to therapy is not immediate and
it can take up to 12 months to determine efficacy.2,6
Finally, for horses with respiratory manifestations of allergies, treatment involves
improving the environment; reducing airway
inflammation; and dilating the airways to
reduce obstruction. Owners should consider
the horse’s “breathing zone”—the two-foot
sphere around his nose. This zone should be
kept as clear of dust and particles as possible.
Keep horses outside the barn and scatter hay
on the ground rather than feeding from a
bale. Acute flare-ups can be treated with corticosteroids (prednisolone or fluticasone) and/
or bronchodilators.3
environmental influences along with treating
the secondary perpetuating factors…” all
while specifically targeting the main cause of
the allergy.6 h
KEY REFERENCES
Prognosis
1. Yu, A.A. Atopy. Proceedings of the Annual Convention of
the AAEP – San Antonio, TX. 2006;52:466-469.
2. Vandenabeele, S. Diagnosis and treatment of
allergic skin diseases in horses. Belgina Equine Practitioner’s Society. 2011. http://www.ivis.org/proceedings/
beps/2011/vandenabeele_en.pdf
3. Oke , S. Managing airway disease. www.TheHorse.
com/15174
4. Rosychuk, R.A.W. A practical approach to the itchy
horse. Proceedings of the 48th British Equine Veterinary
Association Congress. 2009. http://www.ivis.org/proceedings/beva/2009/scientific/7.pdf
5. Thomas, HS. Allergic reactions: types and treatments.
www.TheHorse.com/5262.
6. Yu, A.A. Treatment of equine allergies. Proceedings
of the Annual Convention of the AAEP – San Antonio, TX.
2006;54:469-474.
7. Uckele Health and Nutrition. Equine allergy management through antioxidants and digestive support. http://
www.thehorse.com/ViewArticle.aspx?ID=18150.
8. Stepnik C, Outerbridge CA, White SD, Kass P. Equine
atopic skin disease and response to allergen specific
immunotherapy (ASIT): a retrospective study at the
University of California-Davis (1991-2008) Veterinary
Dermatology 2012; 23:29-36.
Except for anaphylaxis, allergies aren’t usually life-threatening. Successful treatment requires addressing “the patient’s predisposing/
Authored by Stacey Oke, DVM, MSc; reviewed by Stephen D. White, DVM, Dipl.
ACVD
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